1. Scatter half the onion slices in bottom of a 4- or 5-quart slow cooker. Season roast with 1 tsp of the rosemary, the salt, garlic powder and pepper. Add to slow cooker on top of onion.
2. Place carrots, potatoes and remaining onion slices around and on top of roast in slow cooker. Sprinkle with remaining 1 tsp rosemary. Whisk together broth and Worcestershire and pour over vegetables. Cover and slow cook on HIGH for 61/2 hours or on LOW for 8 hours.
3. With a slotted spoon, transfer vegetables to a serving platter. With a large spatula, lift out roast, trying to keep in one piece. Place on top of vegetables and strain liquid into a fat separator or measuring cup. Transfer remaining vegetables to platter.
4. Skim fat from liquid and transfer to a small saucepan. Combine flour and 3 tbsp water in a small bowl. Whisk into saucepan. Bring to a simmer over medium heat. Cook 3 minutes and serve alongside pot roast and vegetables.